The main aim of the ventral fairing of an aircraft is to ensure that it has the necessary useful volume inside it for housing other components such as the main landing gear and the installation of different systems.
The aerodynamic interaction of the ventral fairing with the wing, as well as with the fuselage, is significant both at high subsonic speeds (close to the transonic regime) and at low speeds (close to the incompressible regime). High subsonic speeds are usually defined as being speeds at which the Mach number (Mn) is between 0.7 and 0.95.
The aim of a good aerodynamic design for the outer shape of the ventral is fairing is to minimize the negative interferences between the three interrelated elements (wing, fairing and fuselage), which are sources of aerodynamic drag and of loss of lift of the aircraft, as well as to improve the airflow adherence conditions which a wing-fuselage unit would have without a fairing. In view of the important influence of the fairing on the fluid field around the wing and the fuselage, and therefore on the action of the aircraft, and without at any moment losing sight of the fundamental purpose of the fairing, the external shape thereof must be, as are a large part of engineering products, a compromise solution deriving from the consensus between engineering solutions from different disciplines.
For a given useful interior volume, the shape of the fairing plays an essential role in maximizing the aerodynamic benefit of the aircraft. From an aerodynamic point of view, conventional fairings that were known prior to this invention basically started from the following technical solutions.
A first conventional technical solution corresponds to a simple fairing substantially domed and projecting from below the contour of the fuselage, whose intersection with the aerodynamics of the wing is limited to viscous effects, fundamentally on the upper outer surface of the wing via the upper central zone of the fairing commonly known as the “fillet” (Egbert Torenbeek, “Synthesis of Subsonic Aircraft Design”; Delft University Press, 1976), while the rest (front, lower central and rear zone) of the fairing generally possesses a gentle curvature in the direction of the flow with the aim of minimizing the growth of the limit layer of the actual fairing, through without strongly interacting with the airflow around the wing. An example of such a first conventional technical solution is the ventral fairing of the AIRBUS A330.
A second conventional technical solution corresponds to a more complex fairing which only minimally projects from below the contour of the fuselage, with a greater degree of favorable aerodynamic intersection with the wing, due primarily to the fact that evolution in the direction of the longitudinal axis of the fuselage (X) of the transverse area enclosed by the fairing is highly integrated in the law of global areas of the aircraft (Richard T. Whitcomb, 1952), with the penalisation in the aerodynamic drag of the aircraft being reduced at high subsonic speeds in comparison with the first conventional technical solution. An example of such a second conventional technical solution is the ventral fairing of the AIRBUS A380.
Both conventional solutions share the general shape of the transverse cross-section of their outer surface. In other words, the cross-sections cut through planes perpendicular to the longitudinal axis of the fuselage, which can be referred to as classical and which possess certain distinctive geometric characteristics. So, this “classical” geometric shape presents a symmetric curve with respect to the plane of symmetry of the aircraft, being of variable curvature but always convex as seen from the inside of the volume enclosed by the fairing. In other words, without any changes of the sign of the curvature and without any points of inflection, while the angle (φ) between the cross-section of the fairing and the line corresponding to the cross-section of the lower surface of the wing is an acute angle close to 90 degrees.
Although the second conventional technical solution represents an improvement over the first technical solution from the point of view of aerodynamic drag of the complete aircraft at high subsonic speeds, the volume that it permits to be enclosed by the ventral fairing in its forward part is proportionally significantly less compared to the first conventional technical solution, which implies a consequent drawback when it comes to installing systems in the ventral fairing area.
It is therefore desirable to find a new technical solution for the shape of the ventral fairing which would permit certain aerodynamic characteristics similar to those corresponding to the second conventional technical solution to be combined with a greater useful volume for the installation of systems in its forward part and in its middle part, close to that of a fairing according to the first conventional technical solution.